Unrecognized option: -Xincgc
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.

This is triggered by the final line of /usr/games/freecol, which launches Java, specifying the -Xincgc option (incremental garbage collector). Bionic's default OpenJDK 11 runtime no longer supports this option so the game fails to load.

Removing the "-Xincgc" option from the command line is sufficient to get the game working.